Assessment of Several Interpretation Criteria for Tensile Axial Load Tests on Deep Foundations

Abstract

Axial tensile load tests on deep foundations are increasingly significant because of the expansion in their usage for a myriad of projects requiring support of tensile loads as well as for being a less costly alternative to compression load tests. A variety of interpretation criteria for compression load tests is available, which are often also used for tensile tests, but a few have been proposed specifically for tension load tests. In this study, the performance of seven of the available widely used interpretation criteria was evaluated using a data set of 80 tensile load tests. These criteria are (i) original Davisson, (ii) AASHTO, (iii) New York City building code, (iv) French building code, (v) 5% diameter, (vi) 10% diameter, and (vii) New York University (NYU) compression criteria. Their performance was evaluated for applicability, accuracy, precision, and the diameter and length effects. The AASHTO criterion exhibited the best accuracy and a moderate precision, which is promising. The study also presents efforts to identify a better performing criterion, which yields the introduction of the NYU tension criterion where capacity is defined as that corresponding to the smallest displacement of (i) half the elastic elongation of the pile plus 0.25 in.; (ii) pile head displacement corresponding to the first incidence of pull-out or strain-softening resulting in loss of more than 5% of capacity; and (iii) pile head displacement of 0.5 in., unless modified by the structural engineer of record. The NYU tension criterion achieved a similar accuracy to the AASHTO criterion and a better precision, which is optimal.

Efficacy of Several Design Methods for Predicting the Axial Compressive Capacity of Piles

“…FHWA and Revised Lambda had slopes of 20.002 and 0.004 D, respectively. The results for FHWA were consistent with those obtained in a study (35) comparing 213 piles obtained from the FHWA Deep Foundation Load Test Database (v.2) (31), in which the regression line showed that Q c / Q m increased with pile diameter, suggesting that capacity could be overestimated by 15% for each additional 12in. increase in pile diameter.…”

“…A more recent universal interpretation criterion, known as the NYU criterion, has been developed (32)(33)(34)(35), which exhibits a higher accuracy and precision than Davisson's criterion for interpreting the capacity of a pile from a load test. However, we were unable to employ the NYU criterion, since Davisson's capacity was stored as a numerical value in the databases employed, and the authors did not have access to many of the load settlement curves.…”